His support jumps to 42 percent and lead grows to double digits against Iowa state Sen. Joni Ernst (R), former U.S. Attorney Matt Whitaker (R) and radio host Sam Clovis (R), all of whom hover around 30 percent.

None of the candidates are very well-known and many voters are undecided, however, leaving a great deal of unpredictability in the race. Braley is the best-known candidate in the field, and has a 35 percent favorability rating, with 18 percent seeing him unfavorably. None of the Republicans are known by more than a quarter of general-election voters.

President Obama is very unpopular in a state he won twice, which could hurt Braley's chances. Obama's job approval rating is 39 percent, with 57 percent disapproving.

Republicans are hoping they can compete in the swing state.

The live-caller poll of 1,411 registered voters was conducted from March 5-10 and has a margin of error of plus or minus 2.6 percentage points.